1. Field of the Invention
This invention relates, in general, to electrostatic transfer of marking particle images from a substrate to a receiver member and, more specifically, to a power supply system for establishing a bias voltage on a transfer roller for the purpose of providing the desired electric field for quality electrostatic transfer of images.
2. Description of the Prior Art
In a conventional manner well known in the art of copiers, duplicators, printers, and the like, images are transferred to a receiving member, such as a sheet of paper, by an electrical potential difference between the image and the paper. The required electric field is often provided, especially in color applications, by a transfer roller which has been biased with an electrical voltage to generate the electric field. The power supply which biases the transfer roller is a critical component of a printing machine since the intensity of the electric field is essential to good image transfer and quality reproductions. One of the major problems lies in the fact that the resistivity of the transfer roller is highly dependent upon such things as the humidity, temperature, and roller composition.
Various arrangements have been used in the prior art to ultimately provide the desired electric transfer field by applying a specified voltage to the transfer roller. U.S. Pat. No. 3,837,741, issued Sept. 24, 1974 and assigned to Xerox Corporation, describes three methods of biasing a transfer roller. One method uses a constant current source to develop a voltage across a remote sample of the roller material. The developed voltage controls the output of another voltage power source which biases the transfer roller. A similar method disclosed in the referenced patent uses a roller sample actually mounted on the main roller shaft. The third disclosed arrangement tests the actual transfer roller intermittently at times when the roller is not transferring an image and storing information about the voltage needed to change the actual power source driving the transfer roller when it is functioning. While these methods may offer some advantages, they are determining bias levels based upon measurements which are removed from the actual roller bias application by either time or distance, thus allowing some margin of error.
It is therefore desirable, and it is an object of this invention, to provide a power supply system for biasing a transfer roller which gives an improved control of the electric field produced by the bias on the transfer roller. It is desirable to do this with the least amount of hardware practicable and that feature is also an object of the present invention.